PZ Myers. 2004 Jun 20. Testicle development. <http://pharyngula.org/index/weblog/testicle_development/>. Accessed 2008 Dec 04.
Posted on M00o93H7pQ09L8X1t49cHY01Z5j4TT91fGfr on Sunday, June 20, 2004
Testicle development
The development of the gonad is a peculiar exercise in erecting temporary structures and throwing them away. We actual form three different functional kidneys and three sets of ducts, and recycle some of the kidney ducts to form ducts of the genital system. It really is a urogenital system, and formation of the gonads is intertwined with the formation of the excretory system. All of these changes go on between 5 weeks and 12 weeks of embryonic development in humans.
The first kidney is called the pronephros, and is illustrated in A. These are simple ciliated ducts that open into the coelom, and sweep fluids into a primary nephric or pronephric duct that extends down towards the cloaca. This is a temporary condition; the initial pronephric tubules will begin degenerating shortly.
Before the pronephros degenerates, new tubules form that will contribute to the second kidney, the mesonephros, seen in B. The mesonephric tubules are more sophisticated: they form cup-shaped capsules that that are invested with capillaries, and filter fluids from the blood just like the Bowman's capsules of the adult kidney. The mesonephric ducts drain into the old pronephric duct, which is given a new name: the mesonephric, or Wolffian duct. What isn't illustrated in this diagram is that a second, parallel duct also forms, called the Müllerian duct.
The mesonephros is also doomed to degenerate. The mesonephric duct buds off a small branch (in C) called the metanephric duct. It recruits tissue to form yet more tubules and build a whole new kidney, the metanephric kidney. This is the one you keep. The mesonephros fades away, except for one small piece.
That piece is the duct. In addition to forming a kidney, the mesonephros is the place where other mesodermal cells congregate with the primitive germ cells to form the gonad—the testis in males, the ovary in females (D). The testis and ovary are homologous organs, and both arise in the same place, deep in the body against the dorsal body wall. One early difference is that the female organ adopts the Müllerian duct as its outlet, which will form the Fallopian tubes as she matures. The trusty Wolffian duct is discarded and allowed to wither away.
Males do the reverse, using the pronephric/mesonephric/Wolffian duct to form the epididymus and vas deferens, and the Müllerian duct degenerates.
The testes still have one very important task in front of them: at this point, they're still located deep in the body. During the 28th week, they begin to migrate downward through the abdominal wall, through a set of inguinal canals that open up in front of them (curiously, female embryos also form inguinal canals, but the ovaries spurn them). Once they reach the pelvic floor, though, the testes hang out for another month before actually crawling into the scrotum and completing the descent. This is a harrowing and risky journey for these precious organs. One or both testes fail to complete the process in about 3% of all births, a problem called cryptorchidism, with possible sterility as a result, an increased risk of injury, and an elevated risk of testicular cancer.